regulator: tps6586x: remove regulator-compatible from DT docs

Commit "regulator: deprecate regulator-compatible DT property" deprecated
the use of the regulator-compatible DT property. Update the DT example in
the TPS6586x binding documentation to reflect this.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>